1. Field of the Invention
The present invention relates to mobile radio communications networks and in particular concerns a station for processing transmission signals relating to a cellular mobile radio communications system and to a method for processing these signals.
2. Description of the Related Art
As is known, at the main hierarchical levels of a conventional cellular mobile radio communications network there are the following: a switching center for processing the calls of mobile radio communications traffic and for managing the interface with the public telephone network, base radio stations connected and controlled by the switching center, and mobile stations or terminals in communication with the base radio stations.
In general, the technological choice forming the basis of cellular networks consists in associating the various base radio stations with a respective area or cell within the territory. Moreover, in principle, each cell has associated with it a group of radio channels which may also be reused in another cell, situated at given distance from the first cell and if necessary also adjacent thereto, depending on the type of mobile radio communications system.
According to conventional architecture of cellular networks, the base radio stations, which are located in sites spread over the territory covered by the mobile radio communications system, have the function of performing suitable processing of signals received from the mobile terminals (uplink) or to be transmitted thereto (downlink). In particular, according to conventional architectures, the base radio station houses those apparatus which have the main functions of receiving/transmitting voice or data signals and which perform coding or decoding thereof in accordance with a particular mobile radio communications system used, for example, the system UMTS (Universal Mobile Telecommunication System).
With particular reference to the UMTS systems, in a mobile radio communications network of a specific operator who provides this system, the power of the signals emitted by the mobile terminals is controlled so that the said signals are received by the base radio station with power levels which are substantially the same.
However, it may happen that an antenna of a base radio station does not receive only the signals of a mobile terminal operating within the system of the respective operator, but also signals emitted by an external mobile terminal, i.e. relating to a different operator. In particular, it may happen that this other mobile terminal emits signals on a carrier frequency adjacent to the channel on which the mobile terminal of the subscriber is transmitting. These adjacent signals may interfere with the signal to be received, adversely affecting the reception performance during an uplink connection. In this respect, there exist standards which define the performance characteristics required of a base station receiver for the rejection of the adjacent channels.
A particular type of mobile radio communications network, which has developed in parallel with optical transmission systems, uses optical fiber links between suitable units of the said network.
In this connection, the patent application US-A-2002/0003645 describes a mobile communication network employing a central control unit and a plurality of base stations called “compact” stations and coupled to a base station controller by means of digital optical links, including optical fibers. According to this document, certain specific functions of the network are assigned to the controller and to the central control unit, while the remaining functions are distributed among the compact base stations. In particular, each compact base station comprises the apparatus associated with radiofrequency and, during the uplink connection, receives by means of an antenna the signals transmitted from a mobile station and, after low-noise amplification, performs first analog filtering of a particular frequency. Subsequently, a down-conversion of the frequency of the filtered signal is performed, said signal then undergoing a second analog filtering operation. The resultant signal is again amplified and then transmitted to an analog-digital converter and then to an optical transmitter.
The document EP-A-714218 describes a microcellular communications system, namely a system where the cells have a radius of between 0.5 and 1 km. The system described includes a base structure associated with the microcell and a central station connected to the base structure by means of an optical fiber.
The structure associated with the microcell is provided with an antenna for receiving radiofrequency signals emitted in uplink mode. The signal received, after suitable amplification, is down-converted to a lower frequency and is sent to a low-pass filter which transmits it to an analog-digital converter connected at its output to an optical transmitter. By means of the optical fiber, the optical signal is received by the central station which converts it into an electrical signal to be sent to a digital-analog converter. The resultant analog signal is filtered so as to perform separation of the signals of each channel which are then demodulated.
The patent application JP-A-2001/197012 describes repeater systems for mobile radio communications networks and shows schematically the apparatus of a base station repeater used for the downlink. This base station repeater is connected, by means of an optical fiber, to a repeater for mobile stations, which receives signals from mobile terminals. According to this document, the base station repeater comprises an antenna (intended to receive signals emitted from another base station) connected to a band-pass filter (which in the figures has a passband of 1 MHz) which extracts a component of the signal having a bandwidth W, such as a component with a bandwidth of 200 KHz. The analog signal thus obtained is converted into a digital signal so as to undergo a downsampling operation. The downsampling operation is preceded by a band-pass filtering operation (which in the figures has a passband again of 1 MHz) in order to extract the replica of the signal having a frequency below half the sampling frequency which is to be used in downsampling. After filtering, downsampling and an electro-optical conversion is performed by means of a suitable converter. According to this document, the solution proposed does not require values of the carrier noise (C/N) and dynamic ratio which are high as in the case of conversion into an analog optical signal. Moreover, the use of a low transmission rate means that it is possible to avoid the use of costly electro-optical converters.